Extraction of foundation in offshore platforms

ABSTRACT

The present invention provides a foundation that facilitates the extraction process by minimizing the soil resistance. When the foundation is lowered onto a seabed, it penetrates the seabed and experiences soil resistance therefrom. In particular, the suction generated at the base of the foundation contributes significantly to the soil resistance. The foundation is configured to uniformly distribute pressurized fluid to the base external via a plurality of outlets. The outlets are terminated with an interface layer to allow the transfer of pressurized fluid to the base external and prevent the ingress or seabed sediments into the body. The pressurized fluid released to the base external increases the pore pressure at the base external, thus minimizing the suction at the base.

FIELD OF THE INVENTION

The present invention generally relates to offshore technology, and moreparticularly to the effective extraction of foundations supportingoffshore platforms.

BACKGROUND OF THE INVENTION

Offshore platforms are used extensively for construction of piers andbridges, drilling for natural resources and laying underwater cables.One particular offshore platform is a mobile drilling rig that is mainlyused for oil drilling and gas well operations in water depths up to 120meters. A typical mobile drilling rig has three supporting legs, eachleg being able to extend independently through a jacking-up system. Thebase of each leg comprises a foundation or footing known as a “spudcan”.Nowadays, the foundations of most drilling rigs are equipped with anintegrated water jetting system to assist in the extraction of thefoundations.

When the drilling rig is moved to a desired drilling location, the legsof the drilling rig are extended until the foundations rest upon theseabed. Throughout the entire drilling operation, the foundations maypenetrate deeply into the seabed, thus experiencing soil resistancetherefrom. When the drilling rig needs to be relocated, the legs have tobe extracted from the seabed. During this stage, the buoyancy of thedrilling rig's hull is utilized to overcome the soil resistance exertedon the foundations. The hull is lowered by the jacking-up systemcontrolling each leg to produce the buoyant force. Furthermore, theintegrated water jetting system provides highly pressurized waterthrough the outlets located on the foundation during the extractionprocess. The water jetting system aims to fluidize the soil surroundingthe foundation to facilitate the extraction process. Field observationshave shown that conventional water jetting system is unable to providean effective extraction of the foundation. As such, the extraction ratedepends largely on the capacity of the jacking-up system, which isusually limited. In cases where the foundations experience large soilresistance, the jacking-up system is required to operate for a longerperiod in order to provide sufficient buoyant force to overcome thelarge soil resistance. This delay is a huge factor for the increasingcost in the industry. Furthermore, continuous extraction attempts toovercome large soil resistance may harm the structural integrity of thedrilling rig. Therefore, the extraction process can be considered as oneof the critical phases in drilling rig operations.

U.S. Pat. No. 4,761,096 discloses a universal footing with jettingsystem for marine platforms and structures. More specifically, thedisclosed universal footing comprises a spud-can that functions as afooting base to distribute loadings over a large soil area, and aninternal jetting system to fluidize the soil around the footing.Fluidization of the surrounding soil aims to facilitate the penetrationof the footing into the seabed and also the extraction of the footingfrom the seabed. The disclosed universal footing addresses thedifficulties encountered during the penetration and retrieval of thefooting, but has its drawbacks. For example, the method of distributingpressurized water to fluidize the surrounding soil during extractioncould cause channeling effects. Specifically, the jetting systemprovides pressurized water into the soil through jet nozzle openingslocated on the spud-can. Some of these nozzle openings could createchanneling in the soil once the water pressure is released into thesurrounding soil, thereby resulting in a pressure drop at the remainingnozzle openings. Thus, the disclosed universal footing may not fluidizethe surrounding soil effectively. Furthermore, studies on soilliquefaction had shown that clayey soil does not fluidize. As such, thedisclosed universal footing may not be as effective when deployed inareas with clayey seabed soils.

In addition, the inventor of U.S. Pat. No. 4,761,096 describes theexperimental details of the footing penetration and extraction processesin two publications, namely “A Universal Footing With Jetting” presentedin the Offshore Technology Conference in 1987 and “Effect of Jetting onFooting Penetration and Pullout” presented in the International Offshoreand Polar Engineering Conference in 1995. The experiments disclosed inthe above publications suggest good performance of the universal footingonly in limited conditions. For example, the experiments were performedin a test pit containing fine to medium sand with a surface water ofdepth 0.46 m. The model footing used has a diameter of 0.6 m, asubmerged weight of 90 kg and a penetration depth of 1.52 m. This smallscale model experimented under 1-g conditions does not provide anaccurate simulation of real conditions, wherein the footings deployedhave larger diameters and the footing experiences higher levels ofstress. In field situations, the footing has diameters ranging from 10to 25 m and is deployed in water depths up to 120 m. The penetration ofthe footing can reach up to 20 m in depth. The seabed may also compriseclayey sediments that are less permeable compared to fine and mediumsand. As mentioned above, clayey soil does not fluidize. As such, thedisclosed experiments do not provide a realistic simulation of theactual field conditions.

Therefore, there is an imperative need to have an effective andefficient method for extracting the foundation of an offshore platform.This invention satisfies this need by disclosing an improved foundationthat is able to minimize the soil resistance so as to facilitate theextraction process. Furthermore, the present invention is designed to beeasily implemented into existing offshore platforms. Other advantages ofthis invention will be apparent with reference to the detaileddescription.

SUMMARY OF THE INVENTION

The present invention provides a foundation for use in offshoreplatforms and a system for extraction the foundation penetrated in aseabed.

Accordingly, in one aspect, the present invention provides a foundationcomprising a body having a base, wherein the body is adapted to receivepressurized fluid; and a plurality of outlets disposed on the base,wherein the plurality of outlets are terminated with an interface layerfor allowing pressurized fluid to be released to the base external andpreventing the ingress of seabed sediments into the body, whereby thebody is configured to provide a uniform distribution of the pressurizedfluid to the base external through the plurality of porous outlets,wherein the pressurized fluid released through the plurality of porousoutlets increases the pore pressure at the base external, and therebyminimizing the suction on the base.

In another aspect, the present invention provides a system forextracting an offshore platform foundation penetrated in a seabed,comprising a channel for transferring pressurized fluid to thefoundation; a chamber disposed within the foundation, wherein thechamber is adapted to receive pressurized fluid from the channel; and aplurality of outlets disposed on the base of the foundation, wherein theplurality of outlets are terminated with an interface layer for allowingpressurized fluid to be released to the base external and preventing theingress of seabed sediments into the chamber, whereby the chamberregulates the pressurized fluid received, thereby providing a uniformdistribution of the pressurized fluid to the base external via theplurality of porous outlets, wherein the pressurized fluid releasedthrough the plurality of porous outlets increases the pore pressure atthe base external, thereby minimizing the suction on the base.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments according to the present invention will now bedescribed with reference to the drawings, in which like referencenumerals denote like elements.

FIG. 1 illustrates a cross-sectional side view of a foundation.

FIG. 2 illustrates a bottom view of a foundation comprising a pluralityof outlets according to one embodiment of the present invention.

FIG. 3 illustrates a cross-sectional side view of an outlet with aninterface material according to one embodiment of the present invention.

FIG. 4 illustrated an alternative bottom view of a foundation comprisinga plurality of outlets according to another embodiment of the presentinvention.

FIG. 5 illustrates the contribution of the suction force and theoverlying soil resistance to the net soil resistance of a penetratedfoundation, with respect to the operation period.

FIG. 6 illustrates the behavior of the pore pressure at the base of thefoundation without the application of an external pressure.

FIG. 7 illustrated the behavior of the pore pressure at the base of thefoundation under the application of an external pressure.

DETAILED DESCRIPTION OF THE INVENTION

The present invention may be understood more readily by reference to thefollowing detailed description of certain embodiments of the invention.

Throughout this application, where publications are referenced, thedisclosures of these publications are hereby incorporated by reference,in their entireties, into this application in order to more fullydescribe the state of art to which this invention pertains.

The present invention provides a foundation for an offshore platformthat enables the extraction process to be performed effectively andefficiently. In one embodiment, the foundation 1 comprises an upper body10 and a lower body 20, wherein the upper body 10 and lower body 20 havea frustum shape (see FIG. 1). It should be understood by one skilled inthe art that the foundation 1 can take on other shapes. For instance, acaisson is contemplated. The base 22 of the lower body 20, which alsorefers to as the base 22 of the foundation 1, comprises a plurality ofoutlets 24 and a spigot 26. A chamber 30 is located within the lowerbody 20, wherein the chamber is configured to receive pressurized fluid60, for example water. The pressurized fluid 60 is supplied from asource (not shown) located at the hull of the offshore platform to thefoundation 1 via a pipeline 40, wherein the pipeline is connected to theupper body 10. In specific, the pressurized fluid 60 from the pipeline40 is channeled to the chamber 30 via a plurality of conduits 12,wherein the conduits are disposed in the upper body 10 of the foundation1.

When the pressurized fluid 60 is channeled to the chamber 30, thepressurized fluid is regulated in the chamber and distributed uniformlyover the plurality of outlets 24. The height and base area of thechamber 30 is configured to ensure the proper regulation and uniformdistribution of pressurized fluid 60 over the plurality of outlets 24.Furthermore, the chamber is preferably made from materials that are ableto withstand high pressure, for example steel. In existing foundations1, the internal framework of the lower body 20 is typically composed ofseveral compartments or cubicles. These compartments can beinterconnected to form the chamber 30. Furthermore, the interconnectedcompartments allow equalization of the pressurized fluid 60 received byeach compartment. This ensures the uniform distribution of pressurizedfluid 60 through the plurality of outlets 24.

In a preferred embodiment, the plurality of outlets 24 are disposed atthe base 22 of the foundation 1 in an arrangement illustrated in FIG. 2.Each of the outlets 24 is adapted to terminate with an interfacematerial 50, for example porous metal (see FIG. 3). The interfacematerial 50 acts as a membrane that allows the transfer of pressurizedwater through the outlet 24 to the external of the base 22, and preventsthe ingress of seabed sediments into the chamber 30. Furthermore, theinterface material 50 must be able to withstand the pressure exerted bythe soil surrounding the base of the foundation during the entireoperation period. The interface material 50 is preferably made fromporous materials with a pore size smaller than clay particles, forexample steel. In an alternative embodiment, four triangular outlets 24′are arranged at the base 22 (see FIG. 4). The outlets 24′ are alsoadapted to terminate with the interface material 50. The arrangement andnumber of outlets can be manipulated to provide effective transfer ofpressurized fluid 60 to the external of the base 22.

As afore-discussed, the foundations 1 supporting the structural weightof the offshore platform may penetrate deeply into the seabed when theyare first lowered. The conical spigot 26 at the base 22 of thefoundation 1 provides additional structural stability for the offshoreplatform. As the foundation 1 penetrates deeply into the seabed, soildeposits on top of the foundation 1 and surrounds the base 22 as well.

Centrifuge tests were performed to simulate the actual field conditionsof the foundation 1 penetrating into clayey seabed. The tests utilize acentrifuge model to overcome the limitations of the 1-g model testadopted in the afore-discussed prior art. In FIG. 5, a series of testsconsistently show that the net soil resistance 110 during the extractionof the foundation 1 consists of two main components: the suction 112 ofthe soil at the base of the foundation 1 and the overlying soilresistance 114 at the top of the foundation. In particular, suction 112increases at a much higher rate over a predetermined period of timecompared to that of the overlying soil resistance 114. Over a longeroperation period, suction 112 contributes significantly, up to 60%, tothe net soil resistance 110. The present invention minimizes the suction112 effectively so as to facilitate the extraction process of thefoundation 1.

In general, the water in the pores of soil is known as pore water. Thepressure within the pore water is referred to as the pore pressure.Suction 112 can be defined as negative excess pore pressure with respectto the hydrostatic pressure at the base 22 of foundation 1. Thisnegative excess pore pressure is induced by the extraction of foundation1. Hydrostatic pressure can be referred to as the pore pressure for anygiven depth where there is no water flow. FIG. 6 illustrates thebehavior of the pore pressure at the base 22, wherein pore pressure atthe base increases with depth. Experimental studies on the presentinvention have shown that a portion of the extraction-induced excesspore pressure in the soil surrounding the base 22 of the foundation 1transforms into suction 112. The magnitude of suction 112 depends on thepore pressure prior to the extraction of the foundation 1. Referringagain to FIG. 6, the pore pressure at the base 22 increases during thepenetration of the foundation 1 into the seabed, wherein the foundationstabilizes at a stage 210. Thereafter, the pore pressure starts todissipate during an extended operation period until it reaches a levelproximal to the hydrostatic pressure, as shown in stage 220. When thefoundation 1 is extracted during stage 220, the extraction-inducedexcess pore pressure transforms into suction 112. Continual uplift ofthe foundation 1 is required to overcome the maximum suction 112 atstage 230, followed by the remaining overlying soil resistance 114 untilthe foundation 1 can be fully extracted at stage 240.

The present invention improves the extraction of the foundation 1 byincreasing the pore pressure at the base 22 prior to the extraction, andsupplying pressure throughout the extraction process to compensate forthe suction 112 that would have developed at the base 22. As such, anexternal pressure needs to be supplied at the base 22 to build up thepore pressure at the base external. Preferably, the pore pressure isaccumulated to the maximum level as shown in stage 250 of FIG. 7 whenthe extraction starts. This provides for an initial pressure that issufficient to compensate for the negative pressure (suction 112) that isgenerated during the extraction process.

The pressurized fluid 60 acts as a means for the transferring theexternal pressure to the base 22. Pressurized fluid 60 is first suppliedto the chamber 30 from the pipeline 40 via the plurality of conduits 12.The chamber 30 regulates the pressurized fluid 60 received and providesa uniform distribution of the pressurized fluid through the plurality ofoutlets 24. The pressurized water transferred to the base 22 externalbuilds up the pore pressure of the soil surrounding the base 22.Furthermore, the uniform distribution of pressurized water through theplurality of outlets 24 minimizes any channeling effects that may resultfrom the water jetting system discussed above. The size and arrangementof the plurality of outlets are designed to ensure that the coveragearea of the pressurized fluid 60 released from one outlet overlaps theneighboring outlets. This ensures that the pore pressure build-up overat the entire base 22. Pressure sensors (not shown) can be mounted atthe base 22 to monitor the pressure thereof.

While the present invention has been described with reference toparticular embodiments, it will be understood that the embodiments areillustrative and that the invention scope is not so limited. Alternativeembodiments of the present invention will become apparent to thosehaving ordinary skill in the art to which the present inventionpertains. Such alternate embodiments are considered to be encompassedwithin the spirit and scope of the present invention. Accordingly, thescope of the present invention is described by the appended claims andis supported by the foregoing description.

1. A foundation for use in an offshore platform comprising: a bodyhaving a base, wherein the body is adapted to receive pressurized fluid;and a plurality of outlets disposed on the base, wherein the pluralityof outlets are terminated with an interface layer for allowingpressurized fluid to be released to the base external and preventing theingress of seabed sediments into the body, whereby the body isconfigured to provide a uniform distribution of the pressurized fluid tothe base external through the plurality of porous outlets, wherein thepressurized fluid released through the plurality of porous outletsincreases the pore pressure at the base external, and thereby minimizingthe suction on the base.
 2. The foundation of claim 1 further comprisinga chamber disposed within the body, wherein the chamber is adapted toreceive pressurized fluid, thereby regulating the pressurized fluid toprovide a uniform distribution of the pressurized fluid through theplurality of outlets.
 3. The foundation of claim 1, wherein theplurality of outlets are arranged to ensure that the coverage area ofthe pressurized fluid released from one outlet overlaps the neighboringoutlets.
 4. The foundation of claim 1, wherein the interface layer ismade from porous materials.
 5. The foundation of claim 1, wherein thebase further comprises a pressure sensor to monitor the pore pressure atthe base external.
 6. The foundation of claim 1, wherein the body of thefoundation is a spudcan.
 7. The foundation of claim 1, wherein the bodyof the foundation is a caisson.
 8. A system for extracting an offshoreplatform foundation penetrated in a seabed, comprising: a channel fortransferring pressurized fluid to the foundation; a chamber disposedwithin the foundation, wherein the chamber is adapted to receivepressurized fluid from the channel; and a plurality of outlets disposedon the base of the foundation, wherein the plurality of outlets areterminated with an interface layer for allowing pressurized fluid to bereleased to the base external and preventing the ingress of seabedsediments into the chamber, whereby the chamber regulates thepressurized fluid received, thereby providing a uniform distribution ofthe pressurized fluid to the base external via the plurality of porousoutlets, wherein the pressurized fluid released through the plurality ofporous outlets increases the pore pressure at the base external, therebyminimizing the suction on the base.
 9. The foundation of claim 8,wherein the plurality of outlets are arranged to ensure that thecoverage area of the pressurized fluid released from one outlet overlapsthe neighboring outlets.
 10. The system of claim 8, wherein thefoundation further comprises a plurality of conduits for transferringpressurized water from the channel to the chamber.
 11. The system ofclaim 8, wherein the interface layer is made from porous materials. 12.The system of claim 8, wherein the base of the foundation furthercomprises a pressure sensor to monitor the pore pressure at the baseexternal.
 13. The foundation of claim 1, wherein the foundation is aspudcan.
 14. The foundation of claim 1, wherein the foundation is acaisson.